[Milsurplus] BC-9: A Big Road Block

[howard holden]

Hi dave, connect a small variable capacitor across the tank, relay or 
diode switch controlled, to compensate in the transmit mode. You can 
then tweak it to give the same xmit freq as recv. It'd mean an extra 
switch maybe, but for a real hacker that's not a big issue.

73, Howie WB2AWQ


On 1/20/2017 7:57 AM, David Stinson wrote:
> By using an external tank coil in place of the loop and link-coupling, 
> I'm getting good, sensitive receive and
> am able to get a good Watt of power
> out on 80 Meter CW.  https://goo.gl/photos/SthdKR1rmwxXkZ6z6
>
> I had this dream of making a few contacts with it during Novice Rig 
> Roundup.  But now there's a big problem I should have seen coming.  
> Take a look again at the diagram:
> https://goo.gl/photos/THcWRvYScQfnp5gg9
>
> The RF stage is regenerative through the plate
> resistor, R5.  This reduces the plate voltage on
> the RF tube during receive from 120 VDC to
> 70 VDC.   To transmit, the plate resistor is shorted,
> the full 120 VDC is applied resulting in a strong
> oscillation for transmitting.  And here's the rub:  What happens with 
> a non-
> crystal tube oscillator when one raises the B+
> from 70V to 120V?  Right- the transmit freq
> jumps higher by several KC.
> Some movement is to be expected, but I suspect
> that the original VT-1 didn't move as much as
> the substitute 6AQ5; otherwise the rig wouldn't
> have stayed in service for a decade.
>
> Irreversable changes are absolutely forbidden.
> Not sure how I'm going to address this.  Always
> feel frustrated if I don't get at least one QSO out
> of a rig.   Perhaps something can be done in
> the substitute tube mounting.  I'm open to suggestions?
>
> TNX OM ES 73 DE Dave AB5S
>
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